This invention relates to X-ray mirrors which include multi-layered material.
The ability of multi-layer materials to reflect X-rays with high efficiency depends on the structural quality of the interfaces between the layers. Semiconductor superlattices are one class of multi-layer materials in which the structural and electronic properties of the interfaces are highly perfect, and can be used as X-ray mirrors.
Layered crystalline semiconductor materials with periodic variations in composition in one dimension on the scale of 5-500A commonly known as superlattice structures (Esaki U.S. Pat. No. 3,626,257) have been found to exhibit many novel properties with numerous technological applications. In these materials good semiconducting properties, such as carrier mobilities and minority carrier lifetimes that are comparable to high quality bulk samples of the individual components require that the successive layers be grown epitaxially as single crystal sheets. This requirement is not easy to satisfy in practice and normally limits the composition of crystalline semiconductor superlattices to semiconductors and semiconductor alloys that are lattice matched or nearly lattice matched, and that can be deposited stoichiometrically by the techniques of thin film deposition. The degree of crystallinity of the superlattice material is then limited by the perfection of the substrate material and the ability of the deposited layers to replicate the underlying layers.
A further complication well-known in the art (see for example Dingle U.S. Pat. No. 4,261,771) is that even if the individual semiconductors comprising the intended superlattice material are lattice matched and can be deposited epitaxially as stoichiometric films, standard growth conditions still may not produce a superlattice material with substantially smooth layers since surface diffusion effects and nucleation effects may cause the semiconductor layers to grow in a columnar fashion or otherwise non-uniformly in the lateral direction perpendicular to the plane of the substrate. In this case it is unlikely that layered semiconductor films could be fabricated comprising individual sublayers that are coherent laterally and substantially smooth on the scale of a few interatomic distances. The mechanisms of thin film nucleation and growth are characteristically complex, in that they depend frequently in unexpected ways on the growth conditions such as the substrate temperature and the detailed structure and chemistry of the substrate surface. Thus, in general, one cannot predict what classes of materials or growth conditions can be used to fabricate superlattice structures.
FIG. 1 shows a schematic energy-band diagram of a superlattice structure having undoped crystalline layers where the alternating layers have substantially different compositions and the thickness of each layer is d/2.
Another advance in the field of materials science relating to semiconductor technology in recent years has been the discovery that amorphous semiconductors and insulators can be deposited by a variety of means, reactive sputtering and plasma-assisted chemical vapor deposition (PCVD) being the most popular, in the amorphous state in a substantially defect-free form (PCVD is also known as glow discharge deposition.) By substantially defect-free we mean free of chemically and electrically active coordination defects such as dangling bonds, to a level of better than about 1 defect per 10.sup.3 atoms. This defect-free property manifests itself as a low density of states in the gap, as measured for example, by the optical absorption coefficient for photons with an energy less than the optical bandgap. In one of the more thoroughly studied materials, namely amorphous silicon deposited by plasma assisted CVD from silane gas, the low density of defects is known to result from the passivation of dangling Si bonds by atomic hydrogen. The hydrogen content of these materials depends on the deposition conditions. The materials will be represented by the nomenclature a-Si:H, in the case of amorphous hydrogenated silicon, where the hydrogen content is understood to depend on the detailed nature of the film preparation process.
Prior to the present invention it was not known whether superlattice materials with substantially smooth sub-layers, only a few atomic layers thick (5-500A), can be fabricated from amorphous semiconductors and insulators while simultaneously maintaining their substantially defect-free properties, and whether they can be used as X-ray mirrors.
In view of the non-equilibrium nature of the growth process and the amorphous surface structure of these thin film amorphous semiconductors, prior to the present invention it was not known whether contiguous layers of different composition, a small number of atomic layers thick, can be deposited with long range ordering, that is over lateral distances large relative to the layer thickness, and as a result act as X-ray mirrors. Furthermore, if an amorphous superlattice material could be fabricated it is not known what the nature of the physical properties of this material would be. For example one of the most basic properties of a material namely the electronic energy level positions, can be calculated for crystalline semiconductors in terms of band theory which relies on the nearly perfect periodicity of the crystalline structure. In this case, the material properties can in principle be determined from the relevant electronic states and the band structure.
However, in amorphous materials the electronic energy levels cannot be calculated from the theory in the usual way because of the absence of long range periodicity. Although various alternative approaches have been tried with varying degrees of success, generally the theoretical work has at best succeeded only in describing known properties, without successfully predicting novel properties. Thus, it is not known how to predict the physical and chemical properties of an amorphous semiconductor material in which a periodic potential due to the superlattice structure is imposed in addition to the random atomic potentials of the underlying amorphous network.
The present invention is an X-ray mirror for larger than grazing incidence X-rays made from artificially produced multi-layers. Early attempts to make such X-ray mirrors, based on Bragg reflection from synthetic multi-layer materials, focused on materials in which one of the alternating layers is a high atomic number material such as tungsten and the other layer is a low atomic number material such as Carbon (see for example, T. W. Barbee, Am. Inst. of Phys. Conf. Proc. No. 75, "Low Energy X-ray Diagnostics", ed. D. T. Attwood and B. L. Henke, 1981). The idea is that alternating high atomic number/low atomic number layers will maximize the contrast in optical properties between the layers and hence maximizing the reflectivity for a given number of alternating layers.
However at wavelengths around the carbon, nitrogen and oxygen K shell absorption edges mirrors with alternating layers of Si/SiC, Si/SiN or Si/SiO.sub.2 can have high optical contrast and because of the smooth quality of the plasma CVD deposited films will make excellent mirrors. As pointed out by R. P. Haelbich, A. Segmuller and E. Spiller (Appl. Phys. Lett. 34 184 (1979)) the smoothness of the layers can be as important as the optical contrast between the layers for good specular X-ray reflectivity.
One of the uses of the mirrors described in the present invention is in the optical system of X-ray lasers.